2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and halogenated isostereomers produce a profile of pathologic changes in the skin of humans, monkeys, rabbits and hairless mice characterized by a thickening of the epidermis (acanthosis), hyperkeratosis, and squamous metaplasia of the epithelial lining of the sebaceous glands. A system has been described for the serial cultivation of human epidermal cells in the presence of lethally irradiated mouse 3T3 cells which is ideally suited for studies on the molecular mechanisms of action of TCDD. Cultured human keratinocytes form colonies of stratified squamous epithelium consisting of a basal layer of rapidly dividing cells and superficial layers of cells committed to terminal differentiation. The goals of this research proposal are (i) to characterize the efftect of TCDD on the growth and differentiation of human epidermal cells in culture and (ii) to delineate the role of a specific cytosolic TCDD receptor protein in regulating responsiveness of epidermal cells to TCDD and isosteric halogenated dibenzo-p-dioxins, dibenzofurans, azo- and azoxybenzenes, and biphenyls. Cell differentiation will be assessed by measuring changes in cell size and morphology, keratin patterns in basal and differentiating cells and the synthesis and assembly of cytosolic precursor proteins into cross-linked cornified envelopes. The relationship of these events to the selective gene activation associated with the specific binding and nuclear uptake of TCDD will be studied. Specific cytosolic and nuclear binding of TCDD will be determined by modification of classic steroid hormone receptor assays using 3H-TCDD of high specific activity (36 Ci/mmol). The responses characterized in human keratinocytes will be compared in rat epidermal cells (a tissue refractory to TCDD toxicity in vivo) to determine differences in the two cell types which may determine sensitivity to TCDD and halogenated analogues.